The invention relates generally to pulse width modulated power supplies, and deals more particularly with a pulse width modulated power supply having circuitry to protect against effects of lightening and other excess line voltage conditions.
A standard pulse width modulated power supply comprises a power train transformer having primary and secondary windings. One end of the primary winding is connected to a DC voltage source line and the other end is connected to a semiconductor switch. The switch is repeatedly turned on and off to develop an AC voltage across the primary winding. A half or full wave rectifier bridge is connected across the secondary winding and charges an output capacitor to supply the output DC voltage.
The following prior art technique is used to regulate the output voltage during start up and during normal operating conditions. A differential amplifier receives at one input a fixed reference voltage and at the other input a fraction of the output voltage. The output of the differential amplifier is called an "error voltage" and is proportional to the difference between the output voltage fraction and the reference voltage. The output of the differential amplifier charges a reference capacitor through a current limiting resistor and diode. The error voltage is slow to change because of a feedback capacitor in the differential amplifier and a large resistor in parallel with the reference capacitor. The error voltage is supplied to one input of a comparator. The other input is supplied by the voltage across a timing capacitor which is charged by rectified current from a secondary of another, sense transformer. When the voltage across the timing capacitor just exceeds the error voltage, the comparator resets a flip flop which shuts off the switch and delivery of current to the output capacitor (until the next cycle of a clock). Also, the flip flop activates a transistor to discharge the timing capacitor. Because the error voltage rises gradually as the reference capacitor charges, the output current is limited during start up. Eventually, the reference capacitor charges to a higher, steady state level such that the switch stays on long enough to cause the power transformer to deliver sufficient charging current to the output capacitor to deliver operating current.
When this type of pulse width modulated power supply is subject to a large positive transient voltage spike on the input power source line, the current into the power transformer rises substantially. The timing capacitor will also charge more quickly than under rated voltage, however, not proportionally to the increase of power into the power transformer. The result is that excess power is delivered to the power transformer and excess current is passed through the switch, and the power transformer and switch may be damaged. Another problem occurs after the transient subsides because then the error voltage will be higher than the steady state level due to the excess current during the transient and the large time constant of the error voltage. Also, the timing capacitor will no longer be charged at the increased rate after the transient subsides. Thus, the duty cycle of the switch will be excessive and this could damage the switch and the power transformer.
There are prior art techniques to provide protection from the effects of lightening and excess line voltage conditions by disabling the power transformer switch during the transient. See U.S. Pat. No. 4,731,655. U.S. Pat. No. 4,063,307 discloses a limiting resistor and a zener diode connected from a positive input terminal to the base of an NPN transistor. The collector of the NPN transistor is connected to the base of the power transformer switching transistor. The emitter of the NPN transistor is grounded and the base is provided with a return resistor. If a transient voltage of the input terminal exceeds the zener diode rating, the NPN transistor 114 will be biased on pulling the base of the switching transistor low which turns off the switching transistor for the duration of the transient. While the foregoing techniques may provide the requisite protection during the transient, they will not solve the post transient problem in the foregoing type of pulse width modulated power supply.
A general object of the present invention is to provide a pulse width modulated power supply which avoids prolonged turn on of the primary winding semiconductor switch during and after lightening and other excess line voltage conditions.